There has been widely known a technique for obtaining the vibration state and displacement of a measurement target by irradiating the measurement target with electromagnetic waves and using a Doppler shift of reflected waves that are reflected on the measurement target. Since electromagnetic waves in the microwave to millimeter wave bands have a characteristic of passing through a medium such as a dielectric, attempts by using such electromagnetic waves have been proposed in recent years to detect beating of the heart and respiration that appear as vibrations in the body of a human (examinee) by irradiating the examinee with microwaves. With the use of microwaves, the examinee can be subjected to measurement without touching the body and with clothes on, thereby reducing the burden imposed on the examinee during sensing. An example of such a sensing device using microwaves is a biological signal detecting device disclosed in PTL 1.
The above biological signal detecting device will be described with reference to FIGS. 13 and 14. The biological signal detecting device includes a sensor unit 101, a biological signal extracting unit 102, a distance calculating unit 103, and a biological signal output determining unit 104.
FIG. 13 schematically illustrates the configuration of the sensor unit 101. As illustrated in FIG. 13, a signal transmitted from a local oscillator 301 is split into two signals by a splitter 302. One of the signals is transmitted to a transmission antenna 303, and the other of the signals is further split into two signals by a splitter 308, and the split signals are input to mixers 306 and 307. The two signals obtained by the splitting by the splitter 308 have phases that are different from each other by 90 degrees.
On the other hand, the signal emitted from the transmission antenna 303 is directed toward an occupant and is mainly reflected on the surface of the body of the occupant. Then, the reflection signal that is reflected on the surface of the body is input to a reception antenna 304. At this time, since the surface of the body vibrates due to the movement (including respiration and beating of the heart) of the body of the occupant, the signal transmitted to the occupant receives a Doppler shift. Accordingly, the reflection signal is input to the reception antenna 304 as a reception signal in the state where the phase of the frequency has been modulated.
The reception signal input to the reception antenna 304 is split into two signals by a splitter 305, and the two signals are respectively input to the mixers 306 and 307. The signals input by the splitter 308 are also input to the mixers 306 and 307. The mixers 306 and 307 perform multiplication processing, and baseband signals that have received a Doppler shift are output through low-pass filters 309 and 310. The baseband signals are further subjected to analog-to-digital conversion performed by analog-to-digital (AD) converters 311 and 312 and output as Bi(t) signal and Bq(t) signal. Bi(t) signal and Bq(t) signal are signals having phases that are shifted 90 degrees at an instant.
Bi(t) signal and Bq(t) signal are input to the biological signal extracting unit 102 as illustrated in FIG. 14. The biological signal extracting unit 102 extracts a biological signal, and the distance calculating unit 103 calculates an estimated distance between the sensor unit 101 and the occupant. On the basis of the estimated distance, the biological signal output determining unit 104 sets a threshold and determines whether or not to output the biological signal. Specifically, if a reliability determining unit 408 in the biological signal output determining unit 104 determines that the distance to the occupant is constant or lower than the threshold, the biological signal is output.
Here, Bi(t) signal and Bq(t) signal, which have been converted into digital signals, are signals that have received a Doppler shift, that is, signals whose phases have been modulated by the Doppler frequency, and can be expressed as sine and cosine wave signals, respectively. In the biological signal extracting unit 102, processing is performed in which, after calculation has been performed by a phase signal calculating unit 401, from these two signals, temporal changes in phases, that is, temporal waveforms of heartbeat and respiration are extracted by a heartbeat signal extracting unit 402 and a respiratory signal extracting unit 403, respectively.